<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
       .div-f{
           width: 500px;
           height: 500px;
           background: silver;

           padding-top: 1px;

           margin-top: 100px;
       }
        .div-s{
            width: 100px;
            height: 100px;
            background: red;

            margin-top: 100px;
        }
    </style>
</head>
<body>

<!--  外边距合并:  在垂直方向上, 两个盒子模型的外边距紧相邻, 那么就会产生一种合并现象 -->


    <div class="div-f">
        <div class="div-s">

        </div>
    </div>

</body>
</html>
